Ambient light sensors are used in a variety of applications and devices. For example, ambient light sensors have been provided in lighting fixtures and used to measure an ambient light level of the surrounding environment. The lighting fixtures may use the information about the ambient light level of the surrounding environment to adjust one or more characteristics of the light output thereof. Generally, ambient light sensors include a photodetector of some kind and supporting circuitry for converting measurements from the photodetector into one or more desired signals. FIG. 1 shows conventional ambient light sensor circuitry 10. The conventional ambient light sensor circuitry 10 includes a photodetector 12 coupled in series with a measurement resistor 14 between a supply voltage VCC and ground. Generally, the photodetector 12 is a photodiode and current amplifier that are provided together in a single package. The photodetector 12 includes an input coupled to the supply voltage VCC and an output coupled to an intermediate node 16, and the measurement resistor 14 is coupled between the intermediate node 16 and ground. Control circuitry 18 is coupled to the intermediate node 16 and includes an output 20 for providing an ambient light measurement signal ALMS.
In operation, the photodetector 12 provides a photodetector current I_PD in proportion to the amount of light provided to the photodetector 12. The photodetector current I_PD is provided across the measurement resistor 14, which generates a measurement voltage V_MS at the intermediate node 16. The control circuitry 18 receives the measurement voltage V_MS and performs an analog-to-digital conversion thereon to generate a digital value representative of the measurement voltage V_MS. The digital value is then used to provide the ambient light measurement signal ALMS in a desired format.
Two important performance characteristics of ambient light sensor circuitry are resolution and range. Resolution defines the smallest change in light that is detectable by the ambient light sensor circuitry. Range defines the difference between the smallest light measurement and the largest light measurement that the ambient light sensor circuitry is capable of differentiating between. The size of the measurement resistor 14 and the input voltage range of the control circuitry 18 determine the resolution and range of the conventional ambient light sensor circuitry 10. Due to the limitations of the conventional ambient light sensor circuitry 10, increasing the resolution of the circuitry necessarily decreases the range thereof, and vice-versa.
For example, using a relatively large measurement resistor 14 will allow the conventional ambient light sensor circuitry 10 to differentiate between relatively small changes in light (e.g., 10 foot-candles, 50 foot-candles, etc.), however, the range of the conventional ambient light sensor circuitry 10 will be limited (e.g., 80 foot-candles for a 10 foot-candle resolution and 400 foot-candles for a 50 foot-candle resolution) since the measurement voltage V_MS will quickly reach the maximum input voltage of the control circuitry 18, thereby clipping any measurements that are above the maximum input voltage. The limited range of the conventional ambient light sensor circuitry 10 caused by clipping means that each bit in the digital value used to describe the measurement voltage V_MS after analog-to-digital conversion within the control circuitry 18 defines a more granular light measurement, resulting in a higher resolution. If a relatively small measurement resistor 14 is used, the measurement voltage V_MS is no longer clipped by the maximum input voltage of the control circuitry 18 such that the range of the conventional ambient light sensor circuitry 10 is increased (at the expense of resolution). In other words, while the difference in light described by the smallest digital value used to describe the measurement voltage V_MS after analog-to-digital conversion within the ambient light sensor microcontroller and the largest digital value describing the same may be very large (e.g., 10,000 foot-candles), the resolution is limited to the range divided by the number of bits of the digital value (e.g., 10,000 foot-candles/8 bits=1,250 foot-candles/1 bit).
In many indoor applications, the limited range of the conventional ambient light sensor circuitry 10 is not problematic, due to the fact that indoor light is only provided over a limited range. Accordingly, resolution can be favored over range in indoor applications while losing little to no data. In outdoor applications, range must be favored over resolution to avoid data loss. This may be problematic in applications demanding a high range and resolution. Accordingly, there is a need for improved ambient light sensor circuitry.